It is desirable in an aircraft fuel tank system, to have an overpressure protector to relieve excess pressure and prevent the pressure from reaching a dangerous level.
In a fuel tank system, large pressure differentials may occur when, for example: the aircraft is being refuelled (resulting in high pressure in the tank relative to the atmosphere i.e. positive pressure differential); or the aircraft is making an emergency descent (resulting in low pressure in the tank relative to the atmosphere i.e. negative pressure differential). A common type of overpressure protector comprises a frangible disc, the frangible disc being arranged, during use, to inhibit fluid flow through the overpressure protector, but to fail in response to an excessive pressure differential. Using a frangible disc in overpressure protection is desirable as the overpressure protector can be relatively simple, reliable and lightweight.
Such overpressure protection is typically mounted in the surge tank of the fuel tank system on an aircraft and protects the fuel tank system from experiencing excess pressure differentials in the event that, for example, the flame arrestor becomes blocked. In a known arrangement, a frangible disc is received in a stack pipe in the surge tank. Having the frangible disc located in a stack pipe prevents any overspill fuel leaving the surge tank in the event that the disc fails. The stack pipe may also protect the frangible disc, to some extent, from spurious failures caused by vibrations etc. during flight.
This known arrangement is described in detail below with reference to FIGS. 1 and 2a to 2c. In summary, the stack pipe comprises a base portion to which a lid is tightly bolted via a number of bolts in a circular formation. The frangible disc is held between the base portion and the lid and is sealed by the tight fitting of the lid on the base. The lid is covered with a protective mesh, but is otherwise open to the interior of the surge tank.
In the event that the frangible disc in this known overpressure protector needs replacing (for example if it has failed (ruptured), or if it has exceeded its recommended working life) the stack pipe must be removed from the surge tank by disconnecting rivets on an access panel on the underside of the wing, and removing the stack pipe, together with the access panel, from the aircraft. The complete stack pipe/frangible disc assembly is then unbolted from the access panel. The lid is then removed from the base portion, a new frangible disc is inserted into the stack pipe and the lid is then re-attached to the base portion. The bolts attaching the lid to the base are tightened to form a seal between the frangible disc and the stack pipe. The bolts should be tightened in a predetermined sequence with particular torques so that the new disc is held securely in place, but not subjected to excessive compressive forces. Replacing the frangible disc may therefore be relatively expensive and time consuming, and may result in a dispatch delay. In addition, the frangible disc may be damaged by over-tightening the bolts attaching the lid of the stack pipe to the base, or tightening the bolts on one side of the lid earlier than those on another side of the lid.
Another known type of overpressure protector in a surge tank is a spring-loaded overpressure valve which is arranged to open in the event of an excessive pressure differential. Such a spring-loaded overpressure valve tends to be relatively easy to reset after it has been activated, but it does have a number of drawbacks. For example, it can be unduly complex, vulnerable to jamming and/or unduly heavy.
It is therefore preferable to use frangible discs but it would be desirable to make replacement of the disc simpler and quicker.
Another problem that has been identified with certain forms of overpressure protection relates to the fact that specific type of frangible disc may need to be fitted for specific applications (for example the disc may need to be a particular size or have a particular pressure rating). When a new frangible disc is to be fitted, significant delays can be caused if the replacement disc is not readily available (as may be the case if, for example, the aircraft has landed at a relatively remote airport, but is approaching the end of its permitted flight time for flying with a broken frangible disc).